I created two shared mailboxes and Distribution Groups/security groups with them. Everything works fine except when sendas to extrenal people. I get this resolute.
Your message couldn't be delivered because you weren't recognized as a valid sender. Make sure your sending address is set up correctly and try to send the message again. If the problem continues, please contact your helpdesk.
The following organization rejected your message: CO9EHSMHS028.bigfish.com.
Everyone can see the boxes. They receive emails and send emails to internal people fine. The only issue is when you send to outside address.
Error message one more time.
Your message couldn't be delivered because you weren't recognized as a valid sender. Make sure your sending address is set up correctly and try to send the message again. If the problem continues, please contact your helpdesk.
The following organization rejected your message: CO9EHSMHS028.bigfish.com.